This invention concerns partially fluorinated and fully fluorinated polymers that are substantially transparent to ultraviolet radiation at wavelengths from approximately 187 nanometer to 260 nanometers.
The semiconductor industry is the foundation of the trillion dollar electronics industry. The semiconductor industry continues to meet the demands of Moore""s law, whereby integrated circuit density doubles every 18 months, in large part because of continuous improvement of optical lithography""s ability to print smaller features on silicon. The circuit pattern is contained in the photomask, and an optical stepper is used to project this mask pattern into the photoresist layer on the silicon wafer. Current lithography is done using 248 nm light; lithography with 193 nm light is just entering early production. Alternate methods of lithography that do not use visible or ultraviolet light waves, i.e., the next generation lithographies utilizing X-rays, e-beams or EUV radiation have not matured sufficiently that they are ready to be adopted for production. As use of this new technology develops, there remains a continuing need for improved materials with higher transparencies and greater resistance to radiation damage.
Certain fluoropolymers have already been identified in the art as useful for optical applications such as light guides, anti-reflective coatings and layers, pellicles, and glues. Most of this work has been done at wavelengths above 200 nm where perfluoropolymer absorption is of little concern.
WO 9836324, Aug. 20, 1998, Mitsui Chemical Inc., discloses the use of resins consisting solely of C and F, optionally in combination with silicone polymers having siloxane backbones, as pellicle membranes having an absorbance/micrometer of 0.1 to 1.0 at UV wavelengths from 140 to 200 nm. Data in the literature, together with applicant""s measurements, for fluoropolymers (see Table 1 below) demonstrate that, at least at 157 nm, C and F fluoropolymers have absorbances much larger than A/xcexc=0.1 to 1 as claimed by WO 9836324.
WO 9822851, May 28, 1998, Mitsui Chemicals, Inc., claims the use of photodegradation-resistant, tacky polymers that immobilize dust particles when coated on the inside of a pellicle frame. These tacky materials have compositions consisting largely of low molecular weight xe2x80x94(CF2xe2x80x94CXR) copolymers in which X is halogen and R is xe2x80x94Cl or xe2x80x94CF3. Higher molecular weight polymers such as poly(perfluorobutenyl viny ether), poly[(tetrafluoroethylene/4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole)], poly(tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride), poly(hexafluoropropylene/vinylidene fluoride), or poly(chlorotolyl fluoroethylene/vinylidene fluoride) are added as a minor component to improve creep resistance. It should be noted that all exemplifications of this technology were with poly(chlorotrifluoroethylene) as the low molecular weight adhesive agent and that retention of tackiness after extended UV degradation (illustrated only at 248 nm), not transparency, was the only demonstrated advantage of the claimed formulations.
Japanese Patent 07295207, Nov. 10, 1995, Shinetsu Chem. Ind Co, claims double layer pellicles combining Cytop CTXS (poly(CF2xe2x95x90CFOCF2CF2CFxe2x95x90CF2)) with Teflon(copyright) AF 1600 for greater strength.
U.S. Pat. No. 5,286,567, Feb. 15, 1994, Shin-Etsu Chemical Co., Ltd., claims the use of copolymers of tetrafluoroethylene and five membered cyclic perfluoroether monomers as pellicles once they have been made hydrophilic, and therefore antistatic, by plasma treatment.
European Patent 416528, Mar. 13, 1991, DuPont, claims amorphous fluoropolymers having a refractive index of 1.24-1.41 as pellicles at wavelengths of 190-820 nm. Copolymers of perfluoro(2,2-dimethyl-1,3-dioxole) with tetrafluoroethylene, chlorotrifluoroethylene, vinylidene fluoride, hexafluoropropylene, trifluoroethylene, vinyl fluoride, (perfluoroalkyl)ethylenes, and perfluoro(alkyl vinyl ethers) are cited.
Japanese Patent 01241557, Bando Chemical Industries, Ltd., Sep. 26, 1989, claims pellicles usable at 280-360 nm using (co)polymers of vinylidene fluoride (VF2,), tetrafluoroethylene/hexafluoropropylene (TFE/HFP), ethylene/tetrafluoroethylene (E/TFE), TFE/CF2xe2x95x90CFORf, TFE/HFP/CF2xe2x95x90CFORf, chlorotrifluoroethylene (CTFE), E/CTFE, CTFE/VF2 and vinyl fluoride (VF).
Japanese Patent 59048766, Mar. 21, 1984, Mitsui Toatsu Chemicals, Inc., claims the use of a stretched film of poly(vinylidene fluoride) as having good transparency from 200 to 400 nm.
Many of the fluoropolymers cited in the references above are noticeably hazy to the eye because of crystallinity and would therefore be expected to scatter light to a degree unsuitable for high light transmission and the accurate reproduction of circuit patterns. Poly(vinylidene fluoride), poly(chlorotrifluoroethylene), poly(tetrafluoroethylene/ethylene), commercially available poly(tetrafluoroethylene/hexafluoropropylene) compositions, and poly(ethylene/chlorotrifluoroethylene) are all such crystalline, optically hazy materials. More recent references have thus been directed at Cytop(trademark) and Teflon(copyright) AF because they combine perfluorination with outstanding optical clarity, solubility, and a complete lack of crystallinity. Cytop(trademark) and Teflon(trademark) are less than ideal, however, because the difficulty of the their monomer syntheses make them extremely expensive.
It is an object of the present invention to overcome the difficulties associated with the prior art by providing partially fluorinated and fully fluorinated polymers that are substantially transparent to ultraviolet radiation at wavelengths between 187 and 260 nanometers, especially at 193 nanometers and/or 248 nanometers.
This invention provides An ultraviolet transparent material exhibiting an absorbance/micron (A/micrometer) xe2x89xa61 at wavelengths from 187-260 nm comprising amorphous vinyl copolymers of CX2xe2x95x90CY2, wherein X is xe2x80x94F or xe2x80x94CF3 and Y is H and 0 to 25 mole % of one or more monomers CRaRbxe2x95x90CRcRd where the CRaRbxe2x95x90CRcRd enters the copolymer in approximately random fashion, or 40 to 60 mole % of one or more monomers CRaRbxe2x95x90CRcRd in the case where the CRaRbxe2x95x90CRcRd enters the copolymer in approximately alternating fashion where each of Ra, Rb, and Rc is selected independently from H or F and where Rd is selected from the group consisting of xe2x80x94F, xe2x80x94CF3, xe2x80x94ORf where Rf is CnF2n+1 with n=1 to 3, xe2x80x94OH (when Rcxe2x95x90H), and Cl (when Ra, Rb, and Rcxe2x95x90F). Another useful embodiment within the present wavelength range is 187 to 199 nm.
This invention also provides an ultraviolet transparent material exhibiting an absorbance/micron (A/micrometer) xe2x89xa61 at wavelengths from 187-260 nm comprising amorphous vinyl copolymers of CH2xe2x95x90CHCF3 and CF2xe2x95x90CF2; CH2xe2x95x90CFH and CF2xe2x95x90CFCl; CH2xe2x95x90CHF and CClHxe2x95x90CF2, wherein the ratio of monomers ranges from approximately 1:2 to approximately 2:1; perfluoro(2-methylene-4-methyl-1,3-dioxolane) and perfluoro(2,2-dimethyl-1,3-dioxole); perfluoro(2-methylene-4-methyl-1,3-dioxolane) and vinylidene fluoride in any ratio that gives an amorphous composition; perfluoro(2-methylene-4-methyl-1,3-dioxolane) with tetrafluoroethylene in any ratio that gives an amorphous composition; and the homopolymer of perfluoro(2-methylene-4-methyl-1,3-dioxolane).
This invention further provides pellicles, anti-reflective coatings, optically clear glues, light guides and resists comprising the UV transparent material provided above.
This invention further provides copolymer compositions comprising poly(hexafluoroisobutylene:trifluoroethylene) with 40-60 mole % hexafluoroisobutylene and 60-40 mole % trifluoroethylene and copolymer compositions comprising poly(hexafluoroisobutylene:vinyl fluoride) with 40-60 mole % hexafluoroisobutylene and 60-40 mole % vinyl fluoride.